Formula calculation of pipeline stress

Update:10-04-2020
Summary:

Whether the slotted joint can fully function under load […]

Whether the slotted joint can fully function under load. Regardless of the type of pipe joint, the PVC Pipe Fittings under load exhibits two forms of stress: longitudinal stress and hoop stress. Longitudinal stress is tensile stress and tends to stretch the pipe axially. Fractures caused by longitudinal stress can cause circumferential fractures. The hoop stress is "expansion" and radial expansion, and the potential failure mode is longitudinal splitting. The calculation of the determined stress also shows that the hoop stress will be twice the longitudinal stress:

Hoop stress = (P x OD) / (2 x Tw)

Longitudinal stress = (P x OD) / (4 x Tw),

P is the line pressure, OD is the outer diameter, and Tw is the wall thickness. This means that overstress failure is most likely to occur along the length of the pipe, for example in the weld, rather than on the circumference of the pipe.

Under the same conditions, the reduction in wall thickness will result in an increase in hoop stress. In slotted joints, the coupling housing that engages the groove prevents diameter expansion and strengthens the pipe. This shows that the slotting technique does not generate greater hoop stress and therefore does not weaken the pipeline. Any possible increase in pipe hardness, decrease in tensile strength or decrease in elongation during the pipe grooving process will not affect the compressive capacity of the joint, and changes in pipe materials are comparable to any other cold forming manufacturing operations.

Grooving reduces wall thickness by removing narrow circumferential strips of material from the outer surface. Since the groove is narrow and reinforced by the entire wall thickness of the pipes on both sides of the groove, the hoop stress remains approximately the same. The groove is also reinforced by the connecting key engaged in the groove to prevent it from expanding in the diameter direction. However, the longitudinal stress increases in proportion to the decrease in wall thickness. Therefore, if half of the original wall thickness is retained, the longitudinal stress will double or approximately equal to the hoop stress.

Since the groove depth in a standard-wall pipe is only one-third of the original pipe wall thickness, the hoop stress remains greater than the longitudinal stress. Any overstress failure will continue to occur along the length of the tube (rather than at the groove), indicating that the area of ​​the groove is no weaker than the longitudinal barrel of the tube. Again, this means that the groove will not affect the strength of the joint.

The rated pressure on the grooved mechanical pipe joint is determined based on all the components involved. Without a corresponding joint, the slotted pipe has no rating, and the joint rating is a function of the pipe material and wall thickness. The pipe joint ratings published by each manufacturer are calculated or tested on pipes containing grooves, which means that any potential impact of grooves on the strength of the pipe will be included in the performance level of the coupling.